1. Field of the Related Art
The present invention relates to a lamp string, and particularly, to a flasher string having thyristor diodes coupled with respective flashers in parallel to maintain the circuit loop even when some flashers turn off.
2. Background of the Related Art
The lamp string has been widely used to produce a twinkling light for decoration or celebration of specific festivals, especially at Christmas, as shown in FIG. 1. The lamp string of the prior arts has connecting wires 11 to connect the filaments 13 of the lamps 12 in series to form a loop, which is provided an appropriate voltage source S to turn on the lamps 12. With reference to FIG. 2, the flasher is often used as a switch to turn on or off all the lamps in series so as to create a tinkling light. The flasher 21 comprises a tungsten filament 22, a lamp filament 24 and a metal piece 23. The tungsten filament 22 is joint to the metal piece 23 and normally contacts the lamp filament 24, as shown in FIG. 2A. When the flasher 21 turns on to heat the metal piece 23 by conducting current, the metal piece 23 in contact with the lamp filament 24 is thermally distorted and then escapes from the lamp filament 24 to form an open loop, as shown in FIG. 2B, so that the flasher 21 turns off. Sequentially, the metal piece 23 is cooled down and then returns back to contact the lamp filament 24 so that the flasher 21 turns on again. The above process repeats until the power is cut off. As a result, all the lamps connected in series with the flasher 21 can be turned on or off at a time by turning on or off the flasher 21. Several such lamp strings with different on/off states at the same time may constitute a larger light source to produce a larger area of twinkling light. However, the sense of twinkling light is quite different from the natural star light at night because all the lamps in a specific lamp string should turn on or off simultaneously.
With reference to FIG. 3, another type of flasher 31 (FIG. 3A) with equivalent circuit in FIG. 3B comprises a tungsten filament 32 connected to a metal piece 33 in parallel, which is normally not in contact with a lamp filament 34. When the flasher 31 turns on, the metal piece 33 is thermally distorted towards and then contacts the lamp filament 34. As a result, the two lamp filaments 34, 35, are short circuited and all the current flows through the lamp filaments 34, 35 and the metal piece 33 without any current flowing through the tungsten filament 32 so that the flasher 31 turns off but the current loop is still closed. A lamp string consisting of such flashers 31 may produce a more natural twinkling light source because each flasher 31 can turn on or off independently. However, the current is quite unstable because the short circuited state of the flasher may abruptly increase the current to an extreme level to burn up the lamp filaments, especially when many flashers are short circuited at the same time.
The object of the present invention is to provide a lamp string having a plurality of flashers connected in series and a plurality of thyristor diodes, each thyristor diode connected to a respective flasher in parallel. Each flasher may turn on and off independently to produce a natural-like twinkling light without any impact on the conducting current so that the current is stable during operation.